The present invention relates to certain novel electrodes and their use for electrocatalytically reducing carbon dioxide to carbon monoxide.
Broadly described, the invention comprises an electrode that has a film adsorbed to its surface by reductive electropolymerization of (vbpy)Re(CO).sub.3 Cl (4-vinyl-4'-methyl-2,2'-bipyridine tricarbonylchlororhenium(I)) or its acetonitrile analogue, [(vbpy)Re(CO).sub.3 (MeCN)].sup.+ (4-vinyl-4'-methyl-2,2'-bipyridine tricarbonylacetonitrile rhenium (I)). The poly(vbpy)Re(CO).sub.3 Cl or poly[(vbpy)Re(CO).sub.3 (MeCN)].sup.+ adsorbed onto the electrode, acts as a catalyst for CO.sub.2 reduction to CO. Stability and the reactivity of the polymer film may be increased by co-reductive electropolymerization of either of the aforesaid Re complexes with [(bpy).sub.2 Ru(vpy).sub.2.sup.2+ (bis(4-vinyl-pyridine)bis(2,2'-bipyridine)ruthenium (II)) (vpy is 4-vinylpyridine). A bilayer film assembly consisting of poly-[(bpy).sub.2 Ru(vpy).sub.2 ].sup.2+ as an inner film and the aforementioned copolymer of Ru/Re as an outer layer yields even greater reactivity to CO.sub.2.
Studies have shown that a system based on Re(bpy)(CO).sub.3 Cl (bpy=2,2'-bipyridine) photocatalytically reduces CO.sub.2. Further studies have shown that Re(bpy)(CO).sub.2 X(X=Cl.sup.-, MeCN) and a variety of other complexes, electrocatalytically reduce CO.sub.2 at moderate overvoltages. Work on the thermal and photochemical reactivity of Re.sup.1 (bpy)(CO).sub.3 L(L=H.sup.-.D.sup.-) has provided insight into the mechanism of CO.sub.2 insertion into the Re-H or Re-D bond.
Electrochemically-initiated polymerization of metal complexes of 4-vinyl-4'-methyl-2,2'-bipyridine (vbpy) groups is a versatile technique to form chemically active polymeric films on electrodes. Chemically modified electrodes for electrocatalysis have potential advantages over homogeneous systems in that only small amounts of catalyst are necessary for efficient electrolysis, and enhanced reactivities might be realized by blocking normal solution deactivation pathways.
According to one embodiment of the present invention, carbon dioxide is reduced by potentiostating a Pt or glassy carbon electrode having a Re complex polymeric film, at .ltoreq..sup.- 1.4 V versus a saturated sodium chloride calomel electrode in CO.sub.2 saturated acetonitrile and 0.05-2.0M electrolyte, for example, tetraethylammonium perchlorate or tetrabutylammonium electrolyte. The process is "catalytic" in that each rhenium site reduces many CO.sub.2 molecules and does so at 85-95% faradaic efficiency but most commonly at 92%, and a potential almost 1 V more positive than, for example, a bare Pt electrode (i.e., at this potential the bare electrode is incapable of reducing CO.sub.2) and is thus more energy efficient than a bare electrode.
According to a second embodiment of the present invention, CO.sub.2 is reduced to CO using an electrode as in the first embodiment, however, the polymer film is a copolymer resulting from coreductive electropolymerization of (vbpy)Re(CO).sub.3 Cl or [(vbpy)Re(CO).sub.3 (MeCN)].sup.+ with [(bpy).sub.2 Ru(vpy).sub.2 ].sup.2+. This copolymer film has increased stability and reactivity toward CO.sub.2.
According to a third embodiment of the present invention, CO.sub.2 is reduced to CO using an electrode as in the first embodiment, however, the polymer film is not a single layer but is a two-layer assembly where the inner layer is composed of poly-[(bpy).sub.2 Ru(vpy).sub.2 ].sup.2+ and the outer layer is the Ru/Re copolymer of the second embodiment.